The photorefractive effect, as the term is used herein, is defined to mean that there is an induced refractive index change produced by light followed by a thermal treatment. This is also often called the “photothermal effect” to distinguish it from the classical photorefractive effect which involves light-induced charge redistribution in a nonlinear optical material to produce internal electric fields which, by virtue of the optical nonlinearity, produce local changes in the index of refraction.
Diffractive elements find use in a wide variety of fields. For example, diffractive optical elements are useful for filtering, beam shaping and light collection in display, security, defense, metrology, imaging and telecommunications applications.
One especially useful diffractive optical element is a Bragg grating. A Bragg grating is formed by a periodic modulation of refractive index in a transparent material. Useful utilizations of the this effect are Bragg gratings that reflect wavelengths of light that satisfy the Bragg phase matching condition, and transmit all other wavelengths. Bragg gratings are especially useful in telecommunications applications; for example, they have been used as selectively reflecting filters in multiplexing/demultiplexing applications; and as wavelength-dependent pulse delay devices in dispersion compensating applications.
Bragg gratings are generally fabricated by exposing a photosensitive (photorefractive) material to a pattern of radiation having a periodic intensity. Many photosensitive materials have been used; however, few have provided the desired combination of performance and cost. For example, Bragg gratings have been recorded in germanium-doped silica glass optical fibers, and while such gratings are relatively robust, the fiber geometry and high melting point of the material make these gratings inappropriate for many optical systems. Bragg gratings have also been recorded in photorefractive crystals such as iron-doped lithium niobate. These filters had narrow-band filtering performance, but suffered from low thermal stability, opacity in the UV region, and sensitivity to visible radiation after recording.
Ordinary unpolarized light is made up of many waves that have their electric and magnetic fields randomly oriented, although orthogonal to each other for each wave. If all the electric fields, and consequently also all the magnetic fields, were aligned parallel to one another the light would be linearly polarized. Normal light is considered to be a combination of the two polarizations, vertical and horizontal which is determined by the direction of the electric field. Stated another way, all light is an electromagnetic wave which means that it a wave with an electric field oscillating up and down in one plane, and a magnetic field oscillating up and down in a plane perpendicular to the electric field. The line where those planes cross is the axis along which the wave propagates. A polarizer is anything that allows only light with its electric field in a certain orientation to pass through it.
The use of polarizers is important in telecommunications using optical fibers, particularly single mode optical fibers. Single mode fibers can actually carry the modes with orthogonal orientation. Fibers with circularly symmetric cores cannot differentiate between the two linear polarizations; that is they are degenerate because they are functionally equivalent and cannot be told apart. If the circular symmetry of optical fibers were perfect polarizations would have little impact on telecommunications. However, since fiber symmetry is not perfect the two polarization modes may experience different conditions and travel along the fiber at different speeds. This results in what is called “polarization mode dispersion” which can cause problems in high performance systems. Consequently, it is desirable that only light of having a single polarization be transmitted through optical fibers.
Glass polarizers, the material compositions and the methods for making the glasses and articles made from the glasses have been described in numerous United States patents. Products and compositions are described in U.S. Pat. Nos. 6,563,639, 6,466,297, 6,775,062, 5,729,381, 5,627,114, 5,625,427, 5,517,356, 5,430,573, 4,125,404 and 2,319,816, and in U.S. Patent Application Publication No. 2005/0128588. Methods for making polarizing glass compositions and or compositions containing silver, and/or articles made from polarizing or silver-containing glasses have been described in U.S. Pat. Nos. 6,536,236, 6,298,691, 4,479,819, 4,304,584, 4,282,022, 4,125.405, 4,188,214, 4,057,408, 4,017,316, and 3,653,863. Glass articles that are polarizing at infrared wavelengths have been described in U.S. Pat. Nos. 5,430,573, 5,332,819, 5,300,465, 5,281,562, 5,275,979, 5,045,509, 4,792,535, and 4,479,819; and in additional patents or publications U.S. Pat. No. 6,313,947 and EP 0 719 741. The European patent publication describes a copper-based polarizing glass instead of a silver-based polarizing glass. Additional U.S. patents describing glass optical polarizers and methods of preparing them have been described in, U.S. Pat. Nos. 3,540,793 (Araujo et al.), and 4,304,584 and 4,479,819 (both to Borrelli et al.).
Photosensitive/photorefractive glasses based on the Ce3+/Ag+ redox couple have been proposed as substrates for the formation of diffractive optical elements. For example, U.S. Pat. No. 4,979,975 (Borrelli) discloses a photosensitive glass containing, in weight percent on the oxide basis, about 14-18% Na2O, 0-6% ZnO, 6-12% Al2O3, 0-5% B2O3, 65-72% SiO2 and 0-0.2% Sb2O3, 0.007-0.04% Ag and 0.008-0.005% CeO2, 0.7-1.25% Br and 1.5-2.5% F. In these materials, exposure to radiation (λ˜366 nm) causes a photoreduction of Ag+ to colloidal Ag0, and Ce3+ to Ce4+ which acts as a nucleus for crystallization of a NaF phase in a subsequent heat treatment step. These glasses had a very high absorbance at wavelengths less than 300 nm, making them unsuitable for use with commonly used 248 nm excimer laser exposure systems.
More recently, Elfimov etc. al. in U.S. Pat. Nos. 6,673,497 and 5,586,141 describe a NaF-based photosensitive glass that by the appropriate exposure and thermal development produces a refractive index change in the near infrared that accompanied the development of the NaF phase. The glass composition falls within that composition described in the Borrelli reference cited in the paragraph above. This effect opened the possibilities for applications to optical devices based upon a photorefractive effect, with examples including Bragg gratings and holographic elements. The specific composition disclosed by Glebov et al was very similar to that Borrelli et al. As disclosed above, the important constituents in the glasses are the concentrations of Ce+3 (photosensitizer), Ag+ (photonucleus), and F, with the latter controlling the amount of NaF that can be produced and consequently the maximum amount of possible induced refractive index change. In order to achieve the photosensitive/photorefractive effect in the glass, Glebov's process, like the above described Borrelli reference, involved the exposure to light in the vicinity of 300 nm, or greater, followed by a heat treatment of 520° C. for 2 hours.
While the above patents describe glasses that are either polarizing or photorefractive/photosensitive, none describes a glass that is both polarizing and photorefractive/photosensitive. At the present time, in order to both diffract light and polarize light two separate elements are required. That is, one must use both a diffraction grating element and a polarizer element. The present invention is fulfills the need for a glass composition that can be used to make articles or elements that can perform or exhibit both the photorefractive effect and the polarizing effect within a single element or article.